This invention relates generally to glass-ceramics comprising lithium disilicate and more specifically to glass-ceramics for use in the manufacture of dental restorations and methods of manufacture thereof.
The use of lithium disilicate glass-ceramics for use in dental restorations has been suggested in the prior art. U.S. Pat. No. 4,189,325 to Barret et al. is directed to a glass-ceramic comprising lithium disilicate for use in dental restorations. The glass-ceramic requires the presence of Nb2O5 and Pt as nucleation agents. Barrett et al. introduced dental restorations made from castable lithium disilicate glass-ceramics in the Li2Oxe2x80x94CaOxe2x80x94Al2O3xe2x80x94SiO2 system nucleated by Pt and Nb2O5. According to Barret et al., dental restorations are made by casting a melt into an investment mold, and devitrifying thereafter.
U.S. Pat. No. 4,515,634 to Wu et al. is directed to a castable glass-ceramic composition wherein the glass is melted and cast into a shape and is crystallized after it has been shaped. Therefore, the crystallization process is performed by the technician making the restoration, not the manufacturer of the dental material. Wu set al. suggests one way to improve properties of castable lithium disilicate dental restorations within the same Li2Oxe2x80x94CaOxe2x80x94Al2O3xe2x80x94SiO2 system as described by Barrett et al. is by utilization of P2O5 as a nucleating agent. Both Barrett et al. and Wu et al. describe castable compositions having CaO as an essential ingredient believed to improve chemical durability of the resulting glass-ceramics. Chemical durability is one of the major issues that the Wu and Barrett inventions fail to address. For example, total alkali leaching rates for materials presented in Wu""s examples were four to five times higher than those for commercial dental porcelain.
Castable dental ceramics as described in Barret et al. and Wu et al. employ melting glass ingots supplied by a manufacturer and casting dental articles into a refractory investment mold. Following the casting process, the cast articles are devitrified (crystallized) by the required heat-treatment steps. This process is very similar to casting metals whereby a heat-treatment step follows the casting process to increase hardness and strength.
U.S. Pat. Nos. 5,507,981 and 5,702,514 to Petticrew teach lithium disilicate compositions for use in dental restorations, but the method described therein implies forming glass into the shape of a dental restoration at temperatures much higher than the melting temperature of lithium disilicate and heat-treating the dental restoration after forming to convert the glass into a glass-ceramic.
German Patent Application No. DE19647739 to Schweiger et al. is directed to lithium disilicate compositions for use in dental restorations. The glass-ceramic bodies or blanks used to press dental restorations are defined as xe2x80x9csinterable glass-ceramicsxe2x80x9d which are produced from the starting amorphous glass powder by simultaneous sintering and powder crystallizing, which process is also known as surface crystallization. The glass must be in powder form to be crystallized. Additionally, the lithium disilicate compositions therein require the presence of La2O3, MgO and ZnO.
Many of the lithium disilicate compositions in the prior art require casting of the glass into the desired shape and crystallizing thereafter. The glass must be formed into the finally desired shape and thereafter heat treated to crystallize into a lithium disilicate phase. This may result in structural and other problems, since the microstructure is not formed by the dental materials manufacturer, but by the technician fabricating the dental restoration. Overprocessing by a technician may change the microstructure of the material to something not preferred or desired by the dental materials manufacturer. Moreover, some of the prior art compositions require the forming of the glass-ceramics by surface crystallization, limiting the forming and compositional possibilities of the material.
It is desirable to provide a lithium disilicate glass-ceramic which is pressable or formable after the lithium disilicate is formed. It is beneficial to provide a lithium disilicate glass-ceramic for use in the fabrication of dental restorations wherein crystallization is carried out by the dental materials manufacturer in the most controlled manner. It is beneficial to provide translucent lithium disilicate glass-ceramics having high strength and good presssability.
This invention is directed to lithium disilicate (Li2Si2O5) based glass-ceramics comprising silica, lithium oxide, alumina, potassium oxide and phosphorus pentoxide in addition to other components listed below. The glass-ceramics are useful in the fabrication of single and multi-unit dental restorations including but not limited to orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facing, veneers, facets, implants, abutments, cylinders, and connectors made by a variety of techniques including heat pressing into refractory investment molds produced using lost wax techniques or building and sintering powder onto refractory dies such as in the refractory die/foil technique. The glass-ceramics have good pressability, i.e., the ability to be formed into dental articles by heat pressing, also known as hot pressing, or injection molding, using commercially available equipment and good formability, i.e., the ability to be applied in powder form to a dental model, i.e., on a refractory die, and heated to form a dental restoration.
In accordance with one embodiment directed to the process of making the glass-ceramics, the compositions herein are melted at about 1200xc2x0 to about 1600xc2x0 C. and preferably in the range of about 1300xc2x0 to about 1400xc2x0 C. for a period of time, preferably for about 4 hours and thereafter quenched (e.g., water quenched or roller quenched) or formed into shaped pieces.
The resulting glass is heat-treated to form a glass-ceramic via a one or two-step heat-treatment cycle preferably in the temperature range of about 400xc2x0 to about 1100xc2x0 C. This crystallization heat treatment may comprise a nucleation step and a crystal growth step. Depending on the composition, the first, nucleation step, may be carried out in the range of about 450xc2x0 C. to about 700xc2x0 C. and preferably in the range of about 500xc2x0 C. to about 650xc2x0 C. for about 0.5 to about 4 hours and the second, crystal growth step, may be carried out in the range of about 800xc2x0 C. to about 1000xc2x0 C. and preferably in the range of about 830xc2x0 C. to about 930xc2x0 C. for about 0.5 to about 48 hours. The most preferable heat treatment comprises about a one hour soak at about 645xc2x0 C. and a subsequent four hour soak at about 850xc2x0 C.
The resulting glass-ceramic shaped pieces are provided in a variety of pellets and/or blanks of desired shapes, sizes and structures that may be used for pressing cores or other frameworks or shapes for dental products or restorations or alternatively, used for machining into dental restorations or products. The blank or pellet may be subjected to viscous deformation at a temperature in the range of about 800xc2x0 to about 1200xc2x0 C., and more preferably in the range of about 850xc2x0 to about 950xc2x0 C., and most preferably at less than about 930xc2x0 C., under vacuum and with the application of pressure of between about 2 to about 8 bar (0.2 to 0.8 MPa) and preferably no greater than about 6 bar (0.6 MPa) to obtain a dental restoration. Moreover, it is possible that the blanks may be machined to a dental restoration of desired geometry.
Alternatively, instead of forming into pressable pellets or blanks, the pulverized powder, with or without additives, is used to form a dental restoration using the refractory die technique or platinum foil technique.